The geochemistry and sources of metals and phosphorus in the recent sediments from the Northern Gulf of California

Abstract

The impact of human interventions on water and sediment fluxes from the Colorado River into the Gulf of California is still poorly understood. The erosion of the deltaic structure and the adjacent tidal flats has caused the dispersion and re-deposition of metal-rich clayey mud along the Baja California coast and alternate sediment sources are thought to have gained importance. The sediments in the Northern Gulf of California (NGC) are studied geochemically in order to determine the probable sources and sinks of sediments, metals and phosphorus in the region. Four geochemical factors are identified in the NGC: terrigenous silts and clays, biogenic carbonates, barite and phosphate. Unlike other sediments in the NGC, the Colorado River-derived silts and clays (Fe ≤ 2.8%) are characterised by their high concentrations of labile inorganic-P (269 μg g −1); in these sediments ∼27% of the bulk concentrations of P is found in the exchangeable, Fe and Mn redox-sensitive and Al-oxihydroxide fractions. Apatite is the main P bearing fraction in the sediments, and except in one sample, organic-P is considered to be dominantly refractory. Localised, probably detrital, apatite and barite sources are identified directly adjacent to the Sonora coast. The anomalous enrichment of apatite in one sample with uncommonly high concentrations of labile (non-refractory) P suggests a probable authigenic mineralization in the eastern coast of the NGC. The high abundance of Ba in the Midriff Island region may indicate biogenic sources for this element there. Except for three samples from shallow carbonate bays and the deeper Delfín Basin, the sediments in the NGC contain >90% of Al and Ba and >70 % of Fe, Cu, Cr, Co, Zn and La in residual fractions. The chondrite normalised rare-earth element geochemistry is indicative of typical upper continental crust provenance of the sediments from the NGC.